Paul Kögerler

Giant metal-oxide-based spheres and their topology: from pentagonal building blocks to keplerates and unusual spin systems

Novel synthesis strategies based on the geometrical and topological principles outlined here open up pathways to a new class of spherical clusters with icosahedral symmetry of the type (pentagon)(12)(linker)(30)- also called keplerates -where the centers of the 12 pentagons span an icosahedron and the centers of the 30 linkers an icosidodecahedron. Remarkably, sizing of a spherical molecule is possible for the first time. In addition to their large size of several nanometers, these molecules show unusually high symmetries. When large numbers of paramagnetic metal centers like 30 Fe-III or 20 VO2+ are integrated within their structure, extraordinary spin topologies can be realized on a discrete molecular level. Further functionalization of these systems allows, e.g. to link them forming chains or layers in solid state reactions at room temperature

Archimedean Synthesis and Magic Numbers: “Sizing” Giant Molybdenum-Oxide-Based Molecular Spheres of the Keplerate Type

Pythagorean harmony can be found in the spherical polyoxometalate clusters described here (see illustration for an example of a structure), since there are interesting relationships between the so-called magic numbers (12, 32, 42, 72, 132) relevant for spherical viruses and the number of the building blocks in the cluster. The size of these Keplerate clusters can be tailored by varying the type of connections between the pentagons by means of different spacers.

Polyoxometalate‐Mediated Self‐Assembly of Single‐Molecule Magnets: {[XW9O34]2[MnIII4MnII2O4(H2O)4]}12−

Last night of the POMs: The title compound (X=GeIV) exhibits slow relaxation of magnetization and quantum tunneling with a single-molecule magnetic behavior. Significant structural differences in the [MnIII4MnII2O4(H2O)4]8+ cluster core of the X=SiIV analogue modify the magnetic properties, thereby illustrating how polyoxometalate (POM) ligands can help in the systematic construction of nanoscale magnets.

An Exceptionally Fast Homogeneous Carbon-Free Cobalt-Based Water Oxidation Catalyst

An all-inorganic, oxidatively and thermally stable, homogeneous water oxidation catalyst based on redox-active (vanadate(V)-centered) polyoxometalate ligands, Na10[Co4(H2O)2(VW9O34)2]·35H2O (Na101-V2, sodium salt of the polyanion 1-V2), was synthesized, thoroughly characterized and shown to catalyze water oxidation in dark and visible-light-driven conditions. This synthetic catalyst is exceptionally fast under mild conditions (TOF > 1 × 10(3) s(-1)). Under light-driven conditions using [Ru(bpy)3](2+) as a photosensitizer and persulfate as a sacrificial electron acceptor, 1-V2 exhibits higher selectivity for water oxidation versus bpy ligand oxidation, the final O2 yield by 1-V2 is twice as high as that of using [Co4(H2O)2(PW9O34)2](10-) (1-P2), and the quantum efficiency of O2 formation at 6.0 μM 1-V2 reaches ∼68%. Multiple experimental results (e.g., UV-vis absorption, FT-IR, (51)V NMR, dynamic light scattering, tetra-n-heptylammonium nitrate-toluene extraction, effect of pH, buffer, and buffer concentration, etc.) confirm that the polyanion unit (1-V2) itself is the dominant active catalyst and not Co(2+)(aq) or cobalt oxide.

Structural, Physicochemical, and Reactivity Properties of an All-Inorganic, Highly Active Tetraruthenium Homogeneous Catalyst for Water Oxidation

Several key properties of the water oxidation catalyst Rb(8)K(2)[{Ru(IV)(4)O(4)(OH)(2)(H(2)O)(4)}(gamma-SiW(10)O(36))(2)] and its mechanism of water oxidation are given. The one-electron oxidized analogue [{Ru(V)Ru(IV)(3)O(6)(OH(2))(4)}(gamma-SiW(10)O(36))(2)](11-) has been prepared and thoroughly characterized. The voltammetric rest potentials, X-ray structures, elemental analysis, magnetism, and requirement of an oxidant (O(2)) indicate these two complexes contain [Ru(IV)(4)O(6)] and [Ru(V)Ru(IV)(3)O(6)] cores, respectively. Voltammetry and potentiometric titrations establish the potentials of several couples of the catalyst in aqueous solution, and a speciation diagram (versus electrochemical potential) is calculated. The potentials depend on the nature and concentration of counterions. The catalyst exhibits four reversible couples spanning only ca. 0.5 V in the H(2)O/O(2) potential region, keys to efficient water oxidation at low overpotential and consistent with DFT calculations showing very small energy differences between all adjacent frontier orbitals. The voltammetric potentials of the catalyst are evenly spaced (a Coulomb staircase), more consistent with bulk-like properties than molecular ones. Catalysis of water oxidation by [Ru(bpy)(3)](3+) has been examined in detail. There is a hyperbolic dependence of O(2) yield on catalyst concentration in accord with competing water and ligand (bpy) oxidations. O(2) yields, turnover numbers, and extensive kinetics data reveal several features and lead to a mechanism involving rapid oxidation of the catalyst in four one-electron steps followed by rate-limiting H(2)O oxidation/O(2) evolution. Six spectroscopic, scattering, and chemical experiments indicate that the catalyst is stable in solution and under catalytic turnover conditions. However, it decomposes slowly in acidic aqueous solutions (pH < 1.5).

Terbium Polyoxometalate Organic Complexes: Correlation of Structure with Luminescence Properties†

Light up the POMs: A luminescent lanthanoid complex with polyoxometalate (POM) and organic ligands has been structurally characterized (see picture). Comparison of this octanuclear TbIII complex of 2-picolinate and tungstoarsenate ligands with a dinuclear relative reveals the role of the organic ligands as chromophores, identifies the luminescent Tb centers, and determines the relationship between POM coordination mode and luminescence quenching.

PO43−‐Mediated Polyoxometalate Supercluster Assembly

While the chemistry of discrete and networked high-nuclearity metal clusters continues to evolve steadily, understanding the driving forces and underlying principles governing the aggregation processes of such clusters remains a profound challenge. The imperative need for effective control over their chemical and physical properties—exploited in catalysis, bioinspired chemistry, and materials science—has stimulated considerable interest in the development of more rational and controllable synthetic strategies. In this context, methods based on templating principles promise both enhanced control and greater mechanistic predictability: the nuclearity and geometry of the resulting aggregate is strongly dependent on the size, shape, charge, and the stereoelectronic and coordinative geometric preference of the template. The template effect of anions, in particular, is being increasingly studied in supramolecular organic and coordination-chemistry systems because of its relevance to many chemical and biological processes. While such templated systems may be constructed utilizing noncovalent forces ranging from van der Waals and hydrogen bonding to stronger metal–ligand coordinative interactions, kinetic (template can be removed) and thermodynamic (template becomes integral part of the product) templating phenomena can be differentiated. The latter predominates anion templation in the field of polyoxometalate (POM) chemistry. For example, various anions are enclosed within the central cavities of discrete polyoxovanadates to form templated host–guest complexes of striking structural complementarity. Furthermore, the structure-directing role of anionic species in the condensation of polyoxothiometalate rings was illustrated by S cheresse and co-workers. The same group also reported the construction of a copper-based polyoxotungstate cluster using halide templation. Herein, we describe the formation of a large heterochiral POM architecture, [{a-P2W15O56}6{Ce3Mn2(m3-O)4(m2-OH)2}3(m2-OH)2(H2O)2(PO4)] 47 (1), from multiple molecular components based on the trivacant derivative of the Dawson polyoxotungstate [a-P2W18O62] 6 . Remarkably, the construction of such a highly negatively charged, aggregated POM is mediated by a far smaller anion, phosphate. The complex, isolated as K36Na111·106H2O, was prepared in the course of our efforts to modify the magnetic properties of preformed metal carboxylate clusters by introducing polyoxoanions through ligand competition. We have recently demonstrated that organic bridging ligands on a manganese carboxylate cluster may be partially replaced by polyoxoanions without altering the connectivity of the magnetic cluster core. We now extend this strategy and show that complete ligand substitution can be achieved, thereby giving rise to an allinorganic magnetic cluster based on supporting polyanion ligands. More importantly, an unexpected templating event subsequently organizes these preconceived building blocks into a supercluster, that is, a “cluster of clusters”. Synthesis of 1 is based on a heterometallic high-oxidationstate precursor 2 (Scheme 1) recently reported by Christou and co-workers. The central [Ce3Mn IV 2(m3-O)6] 8+ core of 2

“Molecular Symmetry Breakers” Generating Metal‐Oxide‐Based Nanoobject Fragments as Synthons for Complex Structures: [{Mo128Eu4O388H10(H2O)81}2]20−, a Giant‐Cluster Dimer

The synthesis and manipulation of a huge variety of nanoscaled species of similar chemical nature under one-pot reaction conditions requires access to a potential TMdynamic library∫ of appropriate building blocks.[1a] For instance, by exploiting a detailed knowledge of polyoxometalate chemistry, a variety of discrete clusters (see ref. [1b ± g]) and related extended structures[2] can be formed by the linking of welldefined metal ± oxygen building blocks. These types of compounds have been shown to exhibit unusual topological as well as electronic properties and, furthermore, are interesting for materials science.[3±5] A couple of years ago, we reported wheel-shaped mixed-valence molybdenum clusters of the type {Mo154}, {Mo176}, 6, 7] and {Mo248}; of these, the first two parent species–exhibiting nanometer-sized cavities and therefore presenting fascinating perspectives for a new type of host ± guest chemistry–can now be obtained in high yields in facile syntheses.[8] Herein, we describe for the first time a dimer of two giant clusters, that is, of structurally well-defined covalently linked nanoobjects with a rather high degree of complexity. The dimer contains two elliptical molybdenum oxide based units, linked together by two Eu-O-Mo bonds, each unit incorporates 128 MoVI/V and 4 EuIII centers and includes large fragments of the above-mentioned parent clusters. The interpretation would be that these dimers are formed by EuIII centers acting as symmetry breakers which prevent the corresponding highly symmetrical parent-ring closure.[1b, 6] Of general importance is that in systems showing growth, potential (abundant) agents, such as EuIII centers, can act as TMsymmetry breakers∫ which results in the generation of structural complexity. In any case, it is important to realize that large nanoobject fragments can, in principle, be used as synthons. The ability to connect or assemble clusters in a predefined manner may allow the design of nanoscopic devices using the TMbottom up∫ method (that is, generating large objects from small units). While the TMclassical∫ reduction of an acidified aqueous molybdate solution leads to the blue, wheel-shaped tetraand hexadecameric parent-cluster anions mentioned above,[6] the generation of smaller species requires the presence of electrophiles, such as PrIII ions which increase the curvature by replacing the larger electrophilic {Mo2} -type building units (see below). In the presence of smaller EuIII ions, even ring closure to the parent clusters does not take place, which allows the isolation of compound 1 containing a novel cluster collective. Compound 1 was characterized by single-crystal X-ray structure analysis[9] (including bond valence sum (BVS) calculation to aid in the determination of the (formal) number of MoV centers and protonation sites),[10] elemental analyses ((K), Eu, Mo; see details in ref. [12]), thermogravimetric analysis, redox titration (to aid in the determination of the (formal) number of MoV centers), IR, and EXAFS spectroscopy (Eu-LIII edge,[11] with the option to distinguish in principle between the different Eu centers in the lattice and cluster sites) as well as magnetic susceptibility measurements with a SQUID magnetometer.

Classical and Quantum Magnetism in Giant Keplerate Magnetic Molecules

Complementary theoretical modeling methods are presented for the classical and quantum Heisenberg model to explain the magnetic properties of nanometer-sized magnetic molecules. Excellent quantitative agreement is achieved between our experimental data down to 0.1 K and for fields up to 60 Tesla and our theoretical results for the giant Keplerate species {Mo72Fe30}, by far the largest paramagnetic molecule synthesized to date.

Cucurbit[n]uril−Polyoxoanion Hybrids

The first organic-inorganic hybrid complexes between CB[n] and polyoxometalates not only display a surprisingly high structural complementarity, the right pairing also allows their chemical and physical properties to be coupled, as illustrated by two examples.